The Hubble Space Telescope. This telescope is in space. It takes pictures of things that are too far away to be seen with a regular telescope.

The eight planets of the Solar System, and the sun. Sizes are to scale, but distances are not.

Do you ever wonder about the things in the sky—the Sun, the Moon, the stars? People have been watching the sky for a long time, trying to figure out what is out there. We keep coming up with new ways to learn more about outer space.

Planets are big balls of rock or gas that move around stars. We live on one we call the Earth, which moves around a star we call the Sun. There are at least seven other planets moving around the Sun and a lot of other smaller things as well. All these things together are called a system. The Latin word for the Sun is Sol, so we call this system the Solar System. Far beyond our own Solar System are stars, bodies like the Sun but in some cases much bigger.

Thousands of years ago, a man called Aristarchus said that the Solar System moved around the Sun. Some people thought he was right but many people believed the opposite; that the Solar System moved around the Earth, including the Sun (and even the other stars). This seems sensible, because the Earth doesn't feel as if it's moving, does it?

About 500 years ago another man named Copernicus said the same thing as Aristarchus; that all the planets moved around the Sun and the stars were fixed in space.[1] This time more people agreed but there were still people who thought the opposite. Then, about 100 years later, a man called Galileo began looking at the sky with a new invention: the telescope. He showed that it was very likely that all the planets moved around the Sun. This time even more people thought Galileo may be right and that the Earth really did move around the Sun. Soon, more and more people started using telescopes to study the sky. However there were still some people who thought Galileo was wrong and he was even arrested and taken to court for lying. All the people who believed him began to learn how the planets and the other things in the Solar System moved so they could prove that he was not lying. Thousands of years after Aristarchus, people finally said "Okay, the Earth does move around the Sun". Galileo couldn't be called a liar anymore.[2]

We can use very large telescopes to see what has happened to other stars. We can compare pictures of distant stars with pictures of our own star, the Sun. We live in exciting times because for the first time we have sent people into space and we also have telescopes in space. These telescopes in space take thousands of pictures of the planets, our sun and the distant stars. On Earth people use the photos to learn about all the different things in the Solar System and they try to explain how the Solar System began. We even have a robot on the red planet Mars that moves around and people on Earth tell it where to go and what to photograph. We also want to know what will happen to the Earth and the Solar System in the future.

At the center of the Solar System is the Sun. It is a star, like the billions of other stars in the sky. The other stars are very, very far away, so they look tiny. The Sun is important to us because it gives us heat and energy that allows life. None of the life on Earth could exist without the Sun.[3]

The rest of the things in the Solar System orbit (travel around) the Sun. The planets are the largest of these. Each planet is a little like the Earth. But the planets are also very different from each other.

Many of the planets have moons. A moon orbits a planet. Mercury has no moons[4] and nor does Venus. Earth has one. Jupiter has 63![5]

The planets closest to the Sun are called the inner planets. These are Mercury, Venus, Earth, and Mars. Then comes a big ring of asteroids, chunks of rock much smaller than planets. This ring is called the asteroid belt. Within the asteroid belt, there is a dwarf planet (smaller than a normal planet) named Ceres. Then come the outer planets: Jupiter, Saturn, Uranus, and Neptune. Farther out there are two dwarf planets, Pluto and Eris.

The planets have the names of Roman gods that were worshipped by people thousands of years ago though no one believes in them now. Did you know that some days of the week are also the names of ancient gods? Saturday means "Saturn Day". Thursday means "Thor Day". Thor was a Viking god, the son of Odin. Monday and Sunday simply mean "Moon Day" and "Sun Day". Some of the months are also named after Roman gods. The month of "March" is named after the Roman god "Mars" - he was the god of War!

Beyond the orbit of Neptune is another big ring of things like the asteroids, called the Kuiper belt. Kuiper (said "KYE-per") was the last name of the person who first wrote about it. Most of the things in the Kuiper belt are hard to see through telescopes.

After the Kuiper belt comes the Oort cloud. Scientists think this is where comets come from. It is very far away, many times farther away than Pluto is from the Sun (over a thousand times). It is near the edge of the Solar System.[6] (Yes, "Oort" was the last name of the person who first wrote about it.)

Zodiacal light.

In between all the other things is dust. The pieces of dust are very far apart, but they shine in the light of the Sun. Before dawn, in September or October, they glow in the East. We call this the zodiacal glow, or zodiacal light.[7]

When pieces of space dust hit the Earth's atmosphere, they burn brightly. We call them shooting stars or meteors.

The Sun creates solar wind—a kind of gas that blows away from the Sun into space. This gas travels out past the planets into outer space. The edge, where the solar wind meets the wind from other stars, is called the heliopause. That is about 100 times as far from us as the Earth is from the Sun.[8] Beyond that is a lot of empty space. The nearest star to our Sun is thousands of times farther away than the size of the entire solar system. The Universe is a really huge and empty place![9]

Sir Isaac Newton, the discoverer of gravity. It is said that he came up with the concept of gravity when an apple fell on his head.

Why do all of the planets orbit the Sun? Why do moons orbit planets? Why doesn't the Sun move off and leave the planets behind? The answer to all of these questions has to do with gravity. Gravity is a force that is a property of mass. It pulls things together.

We don't notice the pull from the Sun because it also pulls on the Earth by the same amount. But the Sun's gravity is strong enough to keep the Earth from shooting away. Even though the Earth is going fast, it keeps turning to go around the Sun. It is like they were tied together with an invisible string. In the same way, moons orbit many of the planets. They are kept there by gravity. The Sun itself does not sit still in space. The entire Solar System is orbiting the center of our galaxy. The whole thing stays together because of the force of gravity[10].

Everything is made out of matter. The amount of matter is called mass. Two apples have twice the mass of one apple. The more mass a thing has, the more gravity pulls it and the more its gravity pulls other objects. We don't notice the pull from an apple because it is so much less than the pull from the Earth. If you stand on the ground and let go of an apple, gravity will pull it down towards the center of the Earth. It will hit the ground. If you could throw the apple hard enough at the right angle, it would go into orbit around the Earth. That is how rockets put astronauts into orbit. If you threw the apple really, REALLY hard in the right direction, it would fly away from Earth and never come back; but our arms are not that strong.

The force of gravity from anything is strongest when very close to that thing, and weaker when further from it. Scientists use weight to mean how hard gravity pulls us. Astronauts weigh less on the moon because it has less mass. It does not pull as hard. We actually weigh a tiny bit less on top of a tall mountain than we do in a lower place. This is because we are farther from most of the Earth.[11]

Anyone who looks up at the sky enough can see seven bright objects. These are the Sun, our Moon, Mercury, Venus, Mars, Jupiter, and Saturn. People have known about them for a very long time. Ancient people thought they were related to gods. In Babylon, they named the days of the week after them. Almost everyone was sure that all these things were orbiting the Earth. They did not know we lived in a Solar System.

In about 1500, Nicholaus Copernicus figured out that the planets orbit the Sun. Only the Moon orbits the Earth. But he was afraid to say so for most of his life and only published a full account of his ideas in 1543, the year of his death[12]. Then Galileo Galilei pointed a telescope at the sky. He found moons orbiting Jupiter. He was certain Copernicus was right, and he got in trouble for saying so. It took seventy years to convince scientists that the planets orbit the Sun[13]. Now, almost everyone on Earth understands that we live in a Solar System.

The largest objects at the farthest reaches of the Solar System, even farther than Neptune!

People made better telescopes and found more things in the sky – moons[14], new planets[15], and asteroids[16]. More things, like the dwarf planet Eris, are being found today.[17]

Before the telescope, people explored the sky with their eyes. They saw how the planets seemed to "wander" through the sky. They learned to predict where the Sun, the moon, and planets would be in the sky. They built some observatories -- places for watching the sky. Observe is a more scientific word for watch. They observed the Sun and stars to tell the time of year. In China, they even knew when the moon would block the Sun[18]. Most people thought that celestial bodies could cause war or peace on Earth.[19]

After telescopes were first made, in the early 17th century, people kept making them better. Astronomers saw that planets are not like stars. They are worlds, like the Earth. They could see that some planets have moons.[20] They began to think about what these worlds were like. At first, some thought that the other planets and moons had people or animals living on them. They thought about how it would be to live on these other worlds.[21] Then they made telescopes better, and sent spacecraft into space, and found that there were no plants or animals on the Moon[22] or on Mars.[23]

Now, we can explore by going to some of the other worlds. Twelve Astronauts walked on the Moon about 35 years ago. They brought rocks and dirt back to Earth.[24] Spacecraft flew by Venus, Mars, and the outer planets. The pictures they took showed us a lot of what we know about these worlds.[25] Robots landed on Mars in 1971, 1976, and 1997. They took thousands of pictures of the planets. Two robots, "Spirit" and "Opportunity", are working on Mars right now. They send photos and movies back to Earth. They also check rocks to find out what the rocks are made of.[26]

So far, we have not found any life except on Earth. Maybe tiny one-celled life once lived on Mars. Maybe there is life under the ice on Jupiter's moon Europa. New spacecraft are being planned to look for life on these worlds.[27]

Our Solar System is part of the Milky Way galaxy. Galaxies are big mixes of dust, gas, stars, and other things. Inside our Milky Way galaxy are clouds of dust and gas where stars are born. Our Solar System was created in this kind of cloud. A part of the cloud began to get smaller and less spread out. It formed a big, spinning disk of gas and tiny pieces of dust. This disk was thickest at the middle. The middle slowly collapsed until it became the Sun. We are still trying to learn how the planets were formed. Most scientists think that they were formed from the left over gas and dust.

An artist's illlustration of how the Solar System began.

This is how it could have happened. The rest of the disk continued to spin around the Sun. The tiny pieces of dust hit each other and some of them stuck together, next the bits of dust slowly collected to form grains, these in turn joined to form lumps the size of gravel, then pebbles, and then rocks. The rocks crashed together into mountains. The mountains crashed together to make bigger things. These big things swept up most of the rest of the disk to form the planets, moons, and asteroids.[28]

The Sun got hotter as it collapsed. It began to glow. The temperature at the center reached a million degrees. The Sun started to make a lot of light and heat. This light and heat swept away most of the leftover dust and gas between the inner planets. This light and heat are the sunlight we see and feel every day on Earth. [29]

In another five billion years, the Sun will use up most of its hydrogen fuel. It will enter the final stages of its life. The middle of the Sun will shrink down and become even hotter. The outer layer of the Sun will grow much bigger than it is now. It will form a red giant.

It will be so big that Mercury and Venus, probably Earth and maybe even Mars will be inside it. These planets will burn away. Which planets get destroyed will depend on how much mass the Sun has left.[30] A strong solar wind will blow some of the outer layers of gas away from the Sun. The Sun will have less mass. The Sun’s gravity will be less. All of the planets will move further away from the Sun.[31]

After it has been a red giant for a while, the Sun will start to burn helium. It will shrink down and will not be a red giant any more. It will use the helium up in about a billion years. Then it will become a red giant once again. More gas will blow away for a few hundred thousand years.

A planetary nebula[Note 1] will form. The nebula could last for a few thousand to a few tens of thousands of years. It will glow in the light of the Sun.[32]

At the center, the Sun might shrink into a tiny star called a white dwarf. That kind of star is about the size of Earth. It would take about 100 of these white dwarfs, stacked end to end, to be as wide as the Sun is today. The Sun will not have any more fuel to burn. It will have lots of heat left over and will keep getting cooler and dimmer. Then its light will go out in a hundred billion years from now.[33]

↑Raman, Varadaraja V. (2000). Glimpses of Ancient Science and Scientists. Xlibris Corporation. ISBN073881363X.
page 339 "The Chaldeans ... were also the first to suspect... that the Sun, the moon, the planets and the constellation of stars, all affect human life and destiny.... These beliefs gradually spread .. to Egypt, China, Greece, India, and Rome, for example ... astrology is still very popular."